1. Field of the Invention
The present invention relates to computer buses and particularly to a method and apparatus for facilitating the rapid interconnection of peripheral device interface circuits to a computer bus to establish both power and two-way data transfer. The interconnection is done in a "hot plug" mode in that neither power nor the data transfer are interrupted during the interconnection of a new peripheral device.
2. Description of the Prior Art
Various methods and apparatus for the rapid interconnection of peripheral device interfaces or control circuits to computer buses are known in the art. In an effort to minimize the impact of plugging into a bus, the normal procedure has been to shut down the bus so that new devices would not disrupt data flow on the bus. In contrast the hot plugging concept provides both power and data transfer interconnections without causing interruption of ongoing data transfers on the bus. Hot plugging is found in fault tolerant systems which normally include device or field replaceable unit redundancy coupled through operational comparison and checking logic to ensure correct operation. When a fault is detected an indication of the failing device is provided to service personnel. The failing device is then simply removed from the bus and a replacement device connected. The removal of the failing device and the replacement of a new device are performed without regard to ongoing bus activity. Both the bus architecture and the control device electronics must be carefully designed to achieve this "hot plugging" capability.
The normal control circuits contain electronic microchips mounted on printed circuit (pc) boards. A voltage regulator is included on the p.c. board and circuit interconnections to the bus and for power and data transfer are made via edge connectors. Edge connectors are mounted on a printed circuit board which is plugged into a corresponding receptacle for connection to the bus. Plugging in the board makes electrical contact between the edge connectors and the corresponding bus receptacle and thus provides both power to the electronic components on the pc board and interconnects it with the bus in one operation. For hot plugging, the normal method of interconnection is to increase the length of at least the ground contact on the edge connector, so that a ground contact can be completed prior to the electrical connection of the other contacts for the application of power and the transfer of data signals. Because of the control of the design parameters of the bus architecture and control circuits, hot plugging such circuits into a computer bus ordinarily causes no disruption in the two-way transfer of information on the bus.
Of particular interest is the widening use of the standardized (Small Computer System Interface (SCSI)) bus. The standard defines the mechanical, electrical, and functional requirements for a small computer input/output bus and command protocols to enable attaching small computers with each other and with intelligent peripheral devices such as rigid or flexible disks, magnetic tape drives, printers, optical disks and other direct access storage devices (DASD). Such devices will hereafter be referred to as either SCSI compatible devices or more generally as peripheral devices. The primary objective is to provide host computers with device independence within a class of devices of SCSI to enable a variety of different devices to be added to host computers without requiring modifications to generic system hardware or software.
Several draft specification proposals were presented by the American National Standards for Information Systems, including those dated Dec. 16, 1985 and Mar. 16, 1989. In Section 4.4 of the SCSI specification, there is presented the electrical description for the SCSI bus. The specification allows for the use of TTL technology for the device controllers so that either open collector or tri-state driver devices may be used for interconnecting to the SCSI bus. There are considerable differences in the two types of driver devices since tri-state devices are considerably faster at coming up to operational logic voltage levels than are open collector devices. However, there are many advantages to expanding the types of TTL logic devices which may be connected to the SCSI computer bus. The inclusion of both open collector and tri-state devices for interconnection by a hot plugging method, as will be described later, was discovered to cause operational problems on the SCSI bus. With the widening use of SCSI buses and SCSI compatible devices an attempt was made to use hot plugging techniques on SCSI compatible devices in a fault tolerant environment. The use of SCSI compatible devices in a hot plugging mode would greatly enhance the availability of these devices for fault tolerant operation. However, it was discovered that while many such devices worked with the usual hot plugging technique, some types of SCSI compatible devices disrupted ongoing transfers on the data bus when they were connected.
Thus the problem of hot plugging permitted devices to a SCSI bus has not been encountered before and the prior art methods and apparatus of providing ground contact prior to connecting power and signal to transmit or receive data can in many instances cause disruption of the two-way data flow on the SCSI bus. While the problems were discovered with respect to the two types of TTL devices permitted by the SCSI specification, the problems also exist for hot plugging of NMOs and CMOS implemented peripheral devices, for example, onto a computer bus. There must be a sufficient delay for all peripheral devices to assume a stable high impedance state before data bus connection can begin.